Device in Connection with Heave Compensation

ABSTRACT

The present invention relates to a heave compensator for a riser, particularly a working riser ( 9 ), comprising a telescopic connection ( 10 ) with a first chamber ( 60 ) which is in fluid connection with the interior of the riser and a second chamber ( 62 ) which is connected with a source of pressure fluid. By varying the pressure in the second chamber in response to the movements of the platform, the telescopic connection can be arranged to move in correspondence with the platform&#39;s movements, thereby permitting access to the riser during working operations.

The present invention relates to a device in connection with heave compensation of a pressurised riser extending between a subsea installation and a floating unit, particularly a working riser, comprising a telescopic connection with a first chamber which is in fluid connection with the interior of the riser and a second chamber.

During operations on a subsea well, a floating platform is employed which is held in position by means of anchors or dynamic positioning (DP). Such a platform has to be compensated for movements caused by waves, current and wind. During drilling a heave compensator is employed which keeps the riser under tension during the vessel's movements. In the upper part a telescopic connection is inserted with one part attached to the riser and the other part attached to the platform. The riser is open, with the result that drilling mud that returns up through the riser runs over into a tank. Any volume changes due to the platform's movements are compensated for by the tank having sufficient volume to receive the mud.

When work has to be carried out in a producing well, the riser cannot be open, since the interior of the riser is under pressure, corresponding to the pressure in the well. Thus it has not been common practice to equip working risers with telescopic joints, since the high internal pressure in the riser will attempt to force the telescopic joint into its extreme position, thereby neutralizing the function of the telescopic joint. Moreover, since there is well-pressure in the riser, a pressure safety element, called a surface-mounted wellhead Christmas tree, must be mounted on the top of the riser. It is therefore common practice to suspend the Christmas tree from the platform's derrick and mount the heave compensator for the riser in connection therewith.

Since the Christmas tree is attached to the riser, on account of the wave movements the platform will move relative to the Christmas tree, thereby impeding work on the Christmas tree. Necessary operations, such as the insertion of equipment through the Christmas tree have therefore been performed by personnel being suspended from the derrick in a working harness, which is a hazardous operation that has resulted in many accidents.

In NO patent no. 315 807 a method is described for avoiding such dangerous situations. There the riser is equipped with a telescopic joint. During operations this is locked in one position, with the result that the Christmas tree will move relative to the platform deck. When equipment has to be inserted in the riser, it is firstly lowered until it rests against the seabed. The lock is released and the telescopic joint brought into its central position. The upper part can then be caused to stand still relative to the platform, thereby enabling personnel to perform work in connection with the Christmas tree. The disadvantage of this method is that all work in the well must be interrupted when the telescopic joint is placed in this operating position.

Another disadvantage is that the riser has to be supported at the bottom, which may result in unacceptable bending stresses.

The present invention is based on a principle known from US patent publication no. 2 373 280 wherein a telescopic joint, which is intended to absorb changes in the length of a pipe carrying fluids under pressure, is equipped with oppositely-directed piston surfaces that provide a resultant pressure equal to zero, thus preventing the pressure from causing changes in the length of the pipe.

In NO patent no. 169 027 the use of this principle is proposed in a riser, thereby permitting a telescopic joint to be employed in a working riser with an internal pressure. However, it does not solve the real problem, which is to get the surface tree to stand still relative to the platform while work is in progress.

An object of the present invention is to provide a system that solves at least some of the above-mentioned problems.

A further object is to provide a system where the surface tree can stand still relative to the platform while work is in progress, while at the same time maintaining a substantially constant tension in a main part of the riser.

This is achieved by a device and a method as defined in the attached claims. By means of a solution according to the invention the above-mentioned problems are solved by the second chamber being connected to a source of pressurised fluid with pressure and volume being varied so that heave is actively compensated for the upper part of the riser. The upper part of the riser can thereby be actively regulated relative to the other part of the riser in order to keep the other part under constant tension and/or the upper part can be actively regulated relative to the floating unit during work that requires the surface tree to be stationary relative to the floating unit or other equipment employed for performing the work. The upper part of the riser and the Christmas tree can thereby be accessible under all conditions, including when the tool is located down in the well and can be optimised with regard to their movement in order to avoid unnecessary stresses on the riser system and related equipment.

An advantage of the invention is that it can also be employed for active heave compensation of the riser, thereby avoiding the use of the known tension rods and accumulators for heave compensation.

According to a second aspect of the invention the upper part of the riser may be actively controlled as required with regard to heave and the lower part of the riser may comprise a separate device for heave compensation of this part independent of the upper part of the riser.

A device according to the invention will also comprise, or alternatively be connected with, related equipment that can provide automatic control as a consequence of signals received from one or more sensors, or an arrangement for manual control and/or a combination thereof.

The invention will now be described in greater detail with reference to the accompanying drawings, in which:

FIG. 1 is a schematic view of a typical working riser, and

FIG. 2 illustrates a telescopic joint with a diagram for controlling the telescopic joint.

The riser system illustrated in FIG. 1 is of a type that is normally called an intervention riser or working riser, i.e. it is arranged to be employed during operations in a well after the well is completed and put into production. This may involve, for example, operations for lowering or retrieving equipment to and from the well, stimulating the well with chemical or mechanical agents, etc. A riser of this kind is arranged to withstand high pressure but is normally smaller than the riser employed in drilling operations and usually has an external diameter of 14″.

The configuration illustrated in FIG. 1 is only intended as an example of such types of risers and it will be appreciated that it may comprise more parts, or that other parts may replace those illustrated. The riser is shown attached to the upper part of a subsea Christmas tree 1, which in turn is connected to a wellhead 2 which is fixed in a guide-base 3. The latter forms the foundation of the well 4 and rests on the seabed 5.

From the bottom up the riser system comprises a lower riser package (LRP) 6, an emergency quick disconnect piece (EQDP) 7, a bending joint 8, the pipe 9 and a telescopic joint 10. The riser pipe 9 consists of a number of pipes that are screwed together or interconnected in some other way to form an elongate column. In the telescopic joint 10 there may be provided attachment means for wires 24, which in turn are attached to a tension-based heave compensating system. This is a commonly known arrangement for keeping the riser under tension and implemented in order to avoid excessive loading stresses on the well.

The vessel has a main deck or drill floor 13, which is the primary working area on the vessel and a moon pool 14 through which equipment is lowered to the seabed.

The upper parts of the riser system comprise an adapter connection 15, which forms a transition between the riser 9 and a tension frame 16, which in turn is suspended in the rig's drive gear 17. Inside the tension frame 16 are mounted a surface Christmas tree 18, a surface BOP 19 and a coil pipe injector 20. In the drill floor 13 is mounted a sliding or wear joint 21 through which the pipe 9 is passed in order to avoid damage to the pipe. An umbilical (not shown) leads down to the lower parts of the riser and comprises hydraulic and electrical lines for control of the systems on the seabed.

The vessel further comprises non-illustrated derricks, cranes and other equipment normally found on a vessel. On the vessel there is also located an operations centre with an operator who supervises the operations in the well. In the operations centre there is provided an intelligent control unit that receives and processes data, and is employed for controlling the heave compensating system, as will be described in greater detail in the following section.

According to the invention a number of critical components are provided with meters for measuring their condition. The result of the measurements is transmitted, preferably in real time, to the control unit in the operations centre where the signals are received and processed in the computer.

The critical parameters that require to be measured are primarily the vessel's position, either by measuring the vessel's geographical position by means of a GPS system or by measuring the angle of the riser, or possibly both. Even though the riser's angular deviation from the vertical can be calculated on the basis of the vessel's position, it is desirable to measure it as well, since it provides a verification of the DP system's reliability. In addition, a number of parameters in the heave compensating system are measured, such as the riser's height above the drill floor, the so-called “stick-up” and the change rate for stick-up. In the heave compensating system the piston's position in the cylinder requires to be measured, particularly if it is located near the extreme points of the stroke, together with the change rate, i.e. how fast the piston is moving. In addition, meters are provided in the actual heave compensation.

According to the invention the telescopic joint is designed in such a manner that it can function as active heave compensation, as a replacement for, or in addition to, the standard tension-based heave compensation illustrated in FIG. 1. As illustrated in FIG. 2 the telescopic joint consists in the known manner of an internal telescopic pipe 30 attached to the riser 9 and an external pipe 40 attached to the adapter connector 15 (in FIG. 1). The telescopic pipe 30, and possibly the riser 9, may be equipped with a flange that forms an attachment point for the tension wires 24 (in FIG. 1).

The telescopic pipe 30 has an additional protruding flange with a lower surface 33, an upper surface 34 and an external surface 35. The surface 34 has an area A₅ and the surface 33 has an area A₂. One or more seals 36 are mounted in the surface 35. The telescopic pipe 30 has an internal diameter equal to the riser's 9 internal diameter and has an inner surface 37 and an outer surface 38. One or more holes 39 through the pipe wall are provided in the pipe 30.

The outer part 40 of the telescopic joint comprises an upper part 42 with an inner surface 43 arranged to slide towards the outer surface 38 of the internal telescopic joint. From the upper part 42 there is attached a pipe 44 with an inner surface 45 arranged to slide towards the outer surface 35 of the protruding flange. At the bottom (as viewed in the figure) the pipe 44 is provided with an inwardly directed flange 48 with an inner surface 49 arranged to slide towards the pipe's 30 outer surface 38. The transition between the upper part 42 and the pipe 44 forms a shoulder with a downwardly-facing surface 52 with an area A₄. In a similar manner, the end flange 48 has an upwardly-directed surface 54 with an area A₃. Below the shoulder 52 an opening 56 is provided in the pipe 44. Packers 51 and 53 mounted in the end flange 48 and the upper part respectively provide a seal against the pipe's outer surface 38.

The surfaces 33, 38, 45 and 54 define a first piston chamber 60, which via the holes 39 is connected with the interior of the riser. When the upper part of the telescopic joint moves relative to the lower part, the volume changes in the riser will be compensated for in the chamber 60. The surfaces 38, 34, 45 and 52 define a second piston chamber 62, which via the hole 56 is connected with a line 70, which in turn connects the chamber 62 with a device for regulating pressure and volume in the piston chamber 62. The fluid line 70 leads to a reversible valve 72, which can be regulated between two positions. In the valve's first position the line 70 is connected with a fluid reservoir 80 via a line 74. A controlled throttle valve 75 is advantageously mounted in the line 74. Since the fluid reservoir is at atmospheric pressure, in the said first position the chamber 62 with the valve can thereby be vented to the environment and the heave compensator is in its passive mode.

In the second position the line 70 is connected with a circuit comprising a first line 76 that is connected with the fluid reservoir 80. A controlled pressure-regulating valve, for example a throttle valve 77, is mounted in the line 76. In the second line 78 a pump 82 is mounted. The pump 82 is driven by a motor 83. The pump 82 is supplied with fluid from the reservoir 80 via the line 84. An accumulator 83 is advantageously provided in the line 78 for equalising minor pressure pulses during start-up and stopping of the pump 74. A pressure-controlled check valve 85 is advantageously mounted in the line 78.

A number of sensors are mounted in connection with the heave compensating system. These comprise a pressure sensor 91 in connection with the chamber 60 and a flow meter 92 in the line 70. Furthermore, the control unit 90 receives signals from the DP system 93 and from a heave sensor 94 as previously mentioned. The heave sensor 94 may be an accelerometer, a length meter or a position sensor. The values from these measurements are transmitted to the control unit 90. As previously mentioned, the control unit comprises a programmable unit that processes the said signals and in response thereto controls the pump 82, the control valve 72 and the throttle valves 75 and 77.

The telescopic joint is so designed that it is volume and force-compensated. This is achieved by designing the chamber 60 in such a way that the area A₃ is approximately equal to the area A₁ corresponding to the pipe's 10 internal cross sectional area, which in turn is the theoretical area that is formed by the closed top of the riser, indicated in FIG. 2 by the line 27. Since the riser is at all times under well pressure, it must be closed at the upper end in order to avoid blow-out; in reality this is in the area of the surface Christmas tree, but for the sake of clarity it is illustrated by the dotted line 27.

Changes in fluid volume in the riser are compensated for by the ability of fluid to flow in and out of the chamber 60. The riser is thereby pressure balanced, thus preventing the occurrence of pressure pulses as a result of the volume changes caused by heave movements. When ventilated to the environment, the chamber 62 can act as passive length compensation. In the case of active compensation, fluid is pumped into the chamber in response to the movements in the platform and controlled in the control unit.

In addition, in the event of emergency disconnection or fracture of the riser, the chamber 42 can be controlled so as to cushion a recoil caused by the upwardly-directed force in the riser, thereby preventing the telescopic joint from touching the bottom and causing damage to the platform.

When the system is placed in passive mode, i.e. when the chamber 62 is ventilated to the environment, the heave compensator behaves like a standard type of heave compensator, as is also known from NO 169 027. When the system is run in active mode, it will be possible to choose between synchronising the top of the riser with the platform deck or controlling it so that a tool in the well can stand still in one position, thus permitting operations that require this to be performed in the well. The control system for the telescopic joint may also be programmed to keep the surface Christmas tree at a predetermined height above deck, thus enabling operations to be carried out while the operator is in a safe position. The system may be provided with an alarm that gives a warning when there is a risk of the Christmas tree moving outside of fixed limits.

A further function of the invention is that the traditional tension-based heave compensating system can either be replaced by or combined with the active control of the pressure in the chamber 62. An increase in the pressure in the chamber 62 will give an increased tension in the riser and this can be employed as an alternative heave compensating system. 

1: In combination with a heave compensation device for a pressurized riser which extends between a subsea installation and a floating unit, the riser comprising at least two tubular parts which are connected by a telescopic connection that includes a first tubular member which is connected to the first part, a second tubular member which is connected to the second part and a first chamber which is formed between the first and second members and is connected with the interior of the riser and, the improvement comprising a control system which includes: a second chamber which is formed between the first and second members and is connected with a source of pressure fluid; wherein the source of pressure fluid includes means for adjusting at least one of the pressure and the volume of the fluid in the second chamber to thereby control the movement of the first member relative to the second member. 2: The combination of claim 1, wherein one of the parts of the riser is an upper part which is connected to at least one of a dry Christmas tree, an adaptor connection and a surface BOP. 3: The combination of claim 1, wherein one of the parts of the riser is a lower part which comprises a second heave compensation device that maintains the lower part of the riser under substantially constant tension. 4: The combination of claim 1, wherein the adjusting means comprises a pressure-regulating device. 5: The combination of claim 1, wherein the adjusting means comprises a pump. 6: The combination of claim 4, wherein the adjusting means comprises an intelligent control unit for controlling the pressure-regulating device. 7: The combination of claim 1, wherein the first and second members of the telescopic connection overlap at least partially to thereby form an annulus between them within which the first and second chambers are located. 8: A method for controlling the movement of an upper part of a pressurized riser which is connected to a floating unit and which comprises at least a lower part that is heave compensated by means of tension relative to the floating unit, the lower part and the upper part being connected by a telescopic connection which includes a first chamber in connection with the interior of the riser, the method comprising: providing the telescopic connection with a second chamber in connection with a pressure fluid source; and regulating the supply of pressure fluid to the second chamber in response to at least one of the heave motion of the floating unit and the desired positioning of the upper part relative to the floating unit. 9: The method according to claim 8, wherein the step of regulating the supply of pressure fluid to the second chamber includes regulating the pressure in the second chamber with a pressure regulation device. 10: The method according to claim 9, wherein the pressure regulation device is controlled on the basis of signals received by a data processing unit. 11: The method according to claim 10, wherein the signals comprise data from sensors which measure at least one of the movement of the floating unit and the pressure in the riser. 